Nanoscale Frictional Dissipation into Shear-Stressed Polymer Relaxations

Sliding friction between a silicon tip and a highly cross-linked polyaryletherketone film is studied using friction force microscopy. The friction force as a function of temperature between 150 and 500 K shows distinctive maxima corresponding to alpha and beta polymer relaxations in dynamic mechanical analysis (DMA). In contrast to DMA, the nanoscale friction shows comparable coupling of mechanical energy to both relaxation modes. We report a strong shift in the peak temperatures with applied load. This effect is modeled with an Arrhenius activation by incorporating the applied shear stress in the effective activation energy of the two relaxations. The effect of the stress-shifted relaxation on friction-versus-load experiments is discussed.